12. THE LEAF. 43 



d. The bud-scales are laminae: Lilac, Privet, Abietineae. 



In the last case the bud-scales may be caused to develop into foliage- 

 leaves by cutting off the top of the branch, or removing its leaves, at the 

 time when the bud-scales are developing. 



Cataphyllary leaves are sometimes thickened "so as to serve as 

 depositories for nutritive substances, as in the bulbs of the Onion, 

 Lily, etc. 



5. Bracts and Floral Leaves (Hypsophylls and Sporophylls ; 

 Tig. 27 H). These leaves are generally peculiar in form, texture, 

 and colour ; their morphology is discussed in connection with 

 that of the reproductive organs in 16, as also in Part IV. 



Vernation and Prefoliation. The forms of young leaves and 

 their relative position in the bud, that is their vernation and 

 pre foliation (aestivation and prefloration of floral leaves), require 

 special consideration. 



The vernation (or (estivation) is said to be plane when the leaf is not folded 

 at all; it is conduplicate when the two halves of the leaf are folded 

 inwards face to face on the midrib as on a hinge (e.g. the Bean) : it is 

 plicate when the leaf is folded in numerous longitudinal or oblique pleats 

 (e.g. the Beech); it is crumpled, when the foldings are in all directions (e.g. 

 the petals of the Poppy) ; it is involute, when the lateral halves are rolled 

 inwards towards the mid-rib on the ventral surface (e.g. the Violet) ; it is 

 revolute, when they are rolled inwards towards the mid-rib on the dorsal 

 surface (e.g. the Dock); it is convolute when the whole leaf is rolled up 

 from one lateral margin to the other, so as to form a single roll (e.g. 

 Canna) ; or, finally, it is circinate, when the leaf is rolled longitudinally 

 on itself from the apex downwards (e.g. Ferns). 



The prefoliation (or prefloration) is said to be valvate when adjacent leaves 

 in the bud merely touch by their margins ; when some are overlapped by 

 others it is imbricate ; an intermediate form is that in which one margin 

 of each leaf is directed obliquely inwards, and the other obliquely outward 

 overlapping the inner margin of the next leaf, and is termed contorted or 

 twisted (e.g. petals of the Periwinkle.) 



Valvate prefoliation is only possible in the case of whorled leaves, whereas 

 imbricate prefoliation is characteristic of spirally arranged leaves. A 

 common form of imbricate prefoliation or prefloration is the quincuncial, 

 which occurs in the many dicotyledonous flowers which have a $ calyx ; 

 the five imbricate sepals are so arranged that two are wholly internal, 

 two wholly external, and one partly internal and partly external, connect- 

 ing the outer two with the inner two (see Part IV., Phyllotaxy of the 

 flower). Where the phyllotaxy is distichous (^), the vernation of the 

 leaves is generally conduplicate, and the margins of each older leaf over- 

 lap those of the next younger leaf, giving rise to a form of imbricate 

 prefoliation distinguished as equitant (e.g. Iris) ; in some cases the over- 



